Like a breathalyzer, an ignition interlock measures the alcohol in a person’s system. If that amount exceeds a pre-programmed level, then the interlock temporarily locks the vehicle’s ignition.
To take the test, the driver simply blows into the device. If the sample is good, then the vehicle will start as usual. If alcohol is detected, the driver must wait before testing again. The first time alcohol is detected, the wait time is a few minutes, but if subsequent tests are failed the driver is locked out for increasingly longer periods.
Most states require random re-testing while driving. At times on the road the device will beep, signaling the driver to deliver a breath sample. If alcohol is detected, the ignition interlock will not turn the engine off while the vehicle is in motion. Usually it will signal the driver to stop by blowing the horn and flashing the lights.
The rolling re-test ensures that a driver does not consume alcohol after performing the initial breath test.
The data received from all breath samples are sent to the authority (court/DMV/probation, etc.) that ordered the device to be installed.
A majority of U.S. States and Canadian provinces now use ignition interlocks as an alternative to revoking driving privileges. Interlock devices prevent drunk driving while giving those with DUI convictions the opportunity to work, attend school, and perform other essential tasks. The devices are widely seen as beneficial both to the offender and the public.
Please watch the below video for more information on ignition interlock devices.